1. Field of the Disclosure
The present disclosure relates to a vertical external cavity surface emitting laser (VECSEL) system, and more particularly, to a highly efficient SHG VECSEL system achieving increased wavelength conversion efficiency of a SHG device by reducing laser light's linewidth.
2. Description of the Related Art
VECSELs increase a gain region by adopting an external mirror instead of an upper mirror for a Vertical Cavity Surface Emitting Laser (VCSEL) and obtain a high output power of several to several tens of watts (W) or higher.
FIG. 1 schematically illustrates a typical VECSEL system 10. Referring to FIG. 1, the typical VECSEL system 10 includes a VECSEL device 18, a first mirror 15 obliquely disposed from the VECSEL device 18, and a second mirror 17 reflecting light from the first mirror 15 back into the first mirror 15. The VECSEL system 10 further includes a SHG crystal 16 that is disposed in an optical path between the first and second mirrors 15 and 17 and converts light into light with double the frequency (half the wavelength) and a birefringence filter 14 that is disposed in an optical path between the first mirror 15 and the VECSEL device 18.
The VECSEL device 18 is used to implement a VECSEL system and does not include an upper reflector required by a VCSEL. That is, the VECSEL device 18 includes a distributed Bragg reflector and an active layer. The VECSEL system consists of the VECSEL device 18 and the second mirror 17 that is an external mirror forming a cavity of the VECSEL device 18 together with the reflector of the VECSEL device 18.
The VECSEL device 18 further includes a heat spreader 13 dissipating heat generated in the active layer in order to cool the active layer. The active layer has a resonant periodic gain (RPG) structure with multiple periods of alternating quantum well and barrier layers. A pump beam emitted by a pumping laser (not shown) is absorbed in the quantum well layers so that electrons and holes excited by the pump beam recombine to generate light.
In the above-mentioned structure, the active layer is excited by a pump beam that is emitted by the pumping laser and is incident thereon and emits light of a predetermined wavelength. Laser light pumped by the pump beam and generated in the active layer is reflected by the reflector and then is emitted from the VECSEL device 18 toward the first mirror 15. The laser light reflected from the first mirror 15 passes through the SHG crystal 16 and is incident on the second mirror 17. The laser light has half the wavelength as it passes through the SHG crystal 16.
The wavelength-converted light is reflected from the second mirror 17 and then is emitted through the first mirror 15. The birefringence filter 14 filters out only laser light so that only the laser light of a specific wavelength can resonate.
The SHG crystal 16 has a high wavelength conversion efficiency only in a very narrow wavelength region. In other words, the SHG crystal 16 exhibits wavelength conversion efficiency characteristics over a very narrow Full-Width at Half Maximum (FWHM). For example, when a Periodically Poled Stoichiometric Lithium Tantalate (PPSLT) is used as the SHG crystal 16, the SHG crystal 16 has a high wavelength conversion efficiency for a FWHM of about 0.1 to 0.2 nm.
However, because laser light in an infrared region generated by the VECSEL device 18 exhibits a larger FWHM, the wavelength conversion efficiency of the SHG crystal 16 may be degraded.
For example, in the absence of the birefringence filter 14 and the heat spreader 13, the VECSEL system 10 cannot achieve high wavelength-conversion efficiency for the SHG crystal 16 because laser light emitted by the VECSEL device 18 has a large FWHM of about 1.6 nm.
Thus, the use of birefringence filter 14 and the heat spreader 13 may reduce the FWHM of laser light to some extent. It is known that the FWHM of the laser light passing through the heat spreader 13 and the birefringence filter 14 decreases as the thickness of the birefringence filter 14 and the heat spreader 13 increases.
For example, when a 30-μm-thick heat spreader 13 and a 4-mm-thick birefringence filter 14 are used, the laser light has FWHMs of about 0.29 nm and about 0.35 nm at central wavelengths of 920 nm and 1,064 nm, respectively. When a 500-μm-thick heat spreader 13 and a 4-mm-thick birefringence filter 14 are used, the laser light has FWHMs of about 0.28 nm and about 0.3 nm at central wavelengths of 920 nm and 1,064 nm, respectively. In this way, when the thickness of the birefringence filter 14 is unchanged, the FWHM of laser light decreases as the thickness of the heat spreader 13 increases.
On the other hand, when the thickness of the heat spreader 13 has a fixed value, the FWHM of laser light decreases as the thickness of the birefringence filter 14 increases. For example, when a 500-μm-thick heat spreader 13 and 4-, 5-, and 6-mm-thick birefringence filters 14 are used, the laser light has FWHMs of about 0.29. 0.275, and 0.26 nm at central wavelength of 920 nm, respectively, while it has FWHMs of about 0.3, 0.285, 0.27 nm at central wavelength of 1,064 nm, respectively.
In the absence of the heat spreader 13, when no birefringence filter is used and the thickness of the birefringence filter 14 is 4, 5, and 6 mm, the FWHM of laser light is about 1.6 mm, 0.4, 0.36, and 0.32 nm, respectively. That is, when the VECSEL system 10 does not have the heat spreader 13, the FWHM of laser light decreases as the thickness of birefringence filter 14 increases.
However, when PPSLT is used as the SHG crystal 16, laser light must have a FWHM of about 0.1 to 0.2 nm in order to achieve high wavelength conversion efficiency. Thus, to obtain a sufficiently small FWHM, the thickness of the birefringence filter 14 and the heat spreader 13 must be increased significantly compared to those exemplified above.
The increase in the thickness of the birefringence filter 14 and the heat spreader 13 significantly increases the material costs and the size of the entire VECSEL system 10. Furthermore, this results in high optical loss and low laser output power.
Thus, there is a restriction on reducing the FWHM of laser light by increasing the thickness of the birefringence filter 14 and the heat spreader 13. It is nearly impossible to actually obtain the desired FWHM of laser light by decreasing the thickness of the birefringence filter 14 and the heat spreader 13.